When a vehicle travels on a cant road with an inclination in the width direction of the road (toward a road shoulder) or when the vehicle travels in side winds, the vehicle pulls to the lower side in the inclination direction because of the gravity on the cant road, or the vehicle pulls to the leeward in the side winds because of the wind power although the vehicle intends to travel straight ahead. That is, one-side pull of the vehicle is generated. Hence, to travel straight ahead against the one-side pull of the vehicle, a driver has to rotate a steering wheel to the upper side in the inclination direction or the windward. That is, even during the straight-ahead travel, the driver has to continuously give a steering force in a direction opposite to a direction to which the vehicle pulls (the one-side pull direction).
For example, Japanese Unexamined Patent Application Publication No. 2001-1923 discloses a vehicle steering device that, in the case that a disturbance, which affects a vehicle behavior, is generated (for example, during travel on the cant road or during travel in the side winds), can obtain a disturbance effect value D meeting the degree of effect of the disturbance with respect to the vehicle behavior, and can control a steering assist force Ta in accordance with the disturbance effect value D.
In Japanese Unexamined Patent Application Publication No. 2001-1923, a dead band is provided around the midpoint of a steering torque, and the steering assist force is generated if the steering torque exceeds the dead band, to cancel the disturbance (the inclination of the cant road or side winds).
In addition to the driving style of continuously giving the steering force in the direction opposite to the direction of the one-side pull (in the direction to the lower side of the inclination of the cant road), there is another driving style for the cant road etc. The style is steering the vehicle slightly largely for a short time against the cant road, then releasing the hands from the steering wheel, and if the vehicle body pulls again, steering the vehicle slightly largely again. These steps are repeated (hereinafter, referred to as “instant steering” in this specification). In the case of this driving style, one-side pull response control may not be properly operated with a control method of related art.